The collet used to secure the work in an automatic bar machine, screw machine, or the like, is normally closed by effecting relative longitudinal movement between it and the spindle within which the collet is coaxially mounted. The means that is provided for impressing the force necessary to either slide the spindle over the collet, or to pull the collet into the spindle, is called the "collet closing mechanism." For many years such mechanisms have most commonly comprised an arrangement of chuck levers mounted in a chuck lever fulcrum. Because of structural constraints and the magnitude of forces involved, however breakage has been common in such closing mechanisms and attendant risks to personnel and damage to machinery have been substantial.
Other designs for collet-closing mechanisms have of course also been proposed and adopted. The so-called "ball-chucker" system, for example, utilizes a circular array of ball bearings assembled in a cage behind a displaceable thrust collar. An external, slidably mounted sleeve moves over the ball bearings, forcing them inwardly against the thrust collar to thereby shift it in one direction or the other; this motion is transmitted by appropriate means to effect closure of the collet.
While devices of the kind described may offer advantages over the earlier, lever-operated mechanisms, still they are seriously deficient in certain respects. In particular, because they are subject to extreme wear on mating surfaces, due in part to the minimal areas of contact provided by the spherical bearings employed, the useful life of such mechanisms is relatively short and their practical value is limited.
A collet-operating mechanism that substantially avoids such disadvantages is described and claimed in Nowak; U.S. Pat. No. 4,418,925, issued Dec. 6, 1983. In accordance with the invention described therein an array of wedge members is employed as the force-transmitting means, thus providing extensive areas for contact with the associated bearing surfaces.
Despite the effectiveness and wide acceptance of mechanisms such as that of the Nowak patent, in some instances there is a tendency for the collet to open inadvertently due primarily to the effects of vibrational forces, which promote movement of the operating sleeve. Among the many sources for vibration in automatic screw machines, and the like, are mechanical looseness in spindles and drive means, imbalances in rotating components, free length harmonics set up by rotating bar stock, and cutting forces reflected back through the machine.
As will be discussed more fully below, a current practice that is employed to prevent such accidental release of a collet chuck involves the provision of a block on the machine, so located as to be abutted by the operating sleeve of the closing mechanism in its operative position, to thereby prevent inadvertent retraction. Because such blocks are subjected to constant wear, however, they must be replaced at fairly frequent intervals. Apart from matters of attendant power consumption and hardware costs, the machine down-time required for replacement represents a very significant detriment.
Accordingly, it is an object of the present invention to provide a novel collet-operating mechanism which is inherently adapted to effectively maintain the collet in closed position, and which will readily permit release, as desired.
It is a more specific object of the invention to provide such a mechanism in which the forces inhibiting opening automatically increase with the speed of rotation, thereby affording effectiveness related to dynamic demand.
Other specific objects are to provide a mechanism having the foregoing features and advantages, which is also of relatively uncomplicated design and construction, which affords a long useful life, which is highly effective for its intended purposes, and which is relatively economical and facile to manufacture.